Traditional nonvolatile memory such as “NOR” type Flash relies on a process called channel hot electron injection (CHEi) to charge up a floating gate, such as a polysilicon or nitride layer sandwiched between two oxide layers in a tri-layer stack. The CHEi process occurs near the drain and/or source regions of a MOSFET. During this charging cycle (i.e., a “write cycle”), electrons are accelerated from the source region to the drain region by a horizontal electric field induced by a drain region bias VDS. These electrons then impact ionize an electron-hole pair in the depletion region of the drain. The hole generated recombines in the substrate while the electron may be accelerated further by the vertical field across the gate and be injected into the floating gate. Unfortunately, the CHEi process is rather inefficient since only one in roughly a million electrons end up making the transition through the gate oxide and because the injection is localized to only the drain region.
As integrated circuit device dimensions continue to scale down, the scaling of NOR Flash memory devices requires that the off-state drain current be maintained in ever larger arrays to meet power requirements. This requirement is most affected when a bit line is selected and a high bias placed on it for CHEi programming. The unselected word lines for memory cells that share the selected bit line are kept low to keep their floating gates below threshold, but the high drain field required for CHEi programming of the one selected memory cell causes drain induced barrier lowering leakage in many of the unselected memory cells. To reduce leakage, channel doping remains as high as possible and gate length scaling is limited. The drain turn-off leakage metric therefore limits scaling of the memory cell size and therefore limits how dense the final memory cell array can be made. Accordingly, a method that can maintain and improve the speed of CHEi programming (i.e., improve injection efficiency) but reduce the drain field in unselected memory cells is highly desirable.